As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ABRUPT ENGINE ROUGHNESS NOTED IN-FLIGHT. RETURNED TO AIRPORT, INSPECTION REVEALED THAT 3 EACH CYLINDER STUDS AND 2 EACH THROUGH-BOLTS ON THE NR 4 CYLINDER HAD SHEARED OFF. ALL ENGINE INSTRUMENTS GAVE NORMAL INDICATIONS. ENGINE REMOVED AND SENT TO AN ENGINE FACILITY FOR INSPECTION.